The present invention relates to a turbine housing having an inner housing which is surrounded by an outer housing, in particular for steam turbines. The inner housing and the outer housing each have a first, upper partial region and a second, lower partial region. These partial regions are often designed as separate housing parts. An inner-housing outer surface is positioned opposite an outer-housing inner surface, with a distance between them.
The article xe2x80x9cTemperaturermittlung in Turbinengehxc3xa4usenxe2x80x9d [Temperature determination in turbine housings] by Robert Erich in: Allgemeine Wxc3xa4rmetechnik, Zeitschrift fxc3xcr Wxc3xa4rme-, Kxc3xa4lte- und Verfahrenstechnik, Vol. 9, 1959, pages 163 to 182 deals with the different heating of individual structural elements of a steam turbine during start-up and in the event of load changes during operation. Different heating of this nature causes deformation of the material and stresses which are superimposed on the stresses caused by the steam pressure. The aim of the article is to use calculated and determined temperature distributions to obtain criteria for selecting the steels which are to be used. Then, all the required clearances and gaps can be suitably dimensioned using the thermal expansions which have been determined, which is extremely important when combining two workpieces with different coefficients of expansion. Furthermore, the intention is that it should be possible to use the temperature distributions which have been determined in this way to derive guidelines indicating how known turbines heat up from the cold state and the speed at which load changes should be performed without causing creep phenomena in the material as a result of excess loads.
The object of the present invention is to keep distortion of the turbine housing during cooling at a low level.
The invention is based on the recognition that, in a steam turbine which has a turbine outer housing and a turbine inner housing or a guide-vane carrier, temperature differences arise after the turbine has been switched off and between the respective housings or between the outer housing and a guide-vane carrier. These temperature differences can lead to distortion of both housings, which in turn leads to undesirable stresses and to clearances being bridged. This means that under adverse conditions turbine blades may strip the housing and thus cause stripping damage. Because of its appearance, the distortion which occurs during natural cooling of the outer housing is also referred to by the term xe2x80x9cbowingxe2x80x9d.
The turbine housing has an inner housing which is surrounded by an outer housing. In the following text, the term xe2x80x9cinner housingxe2x80x9d is also understood as meaning a guide-vane carrier. The inner housing and the outer housing are each divided into a first, upper partial region and a second, lower partial region. An inner-housing outer surface and an outer-housing inner surface are positioned opposite one another, with a distance between them. The inner-housing outer surface and the opposite outer-housing inner surface, at least in a part of their respective first partial regions, are designed in such a way that, in those regions, they exhibit a lower heat transfer through radiation than at least in a part of their respective second partial regions. This makes it possible, after the turbine has been switched off, to prevent the outer housing from cooling too quickly with respect to the inner housing. This is because if the inner-housing outer surface were to exhibit approximately the same heat transfer to the opposite outer-housing inner surface in the first and second partial regions, a not inconsiderable rising flow would be initiated in the upper region in the intermediate space formed between the two opposite surfaces. This rising flow would lead to a higher introduction of heat into the first, upper partial region of the outer housing. According to the invention, with cooling by natural convection, the lower heat transfer in the first partial region makes it possible to provide temperature compensation, so that the temperature difference between the first, upper partial region and the second, lower partial region can be considerably below previously known temperature differences of over 50 Kelvin with natural cooling and without additional measures.
A first, particularly advantageous configuration for forming a lower heat transfer as a result of radiation involves the inner housing, in the first partial region on the inner-housing outer surface, having a first emission coefficient which is lower than a second emission coefficient of the second partial region on the inner-housing outer surface. For temperature compensation, it has proven advantageous if the first emission coefficient is below 0.5 and the second emission coefficient is above 0.5. This is also to be considered dependent on the material used for the inner and/or outer housing. In fact, to avoid stresses in the housings themselves, both housings usually consist in each case of the same type of material. The emission coefficient of the material in question can still be decisively influenced by suitable surface processing, for example by controlled roughening of the surface, in order in this way to obtain a suitable emission coefficient. Preferably, the surface is processed in such a way that the material""s properties, such as strength and resistance to corrosion, are at most affected to an insignificant extent.
A refinement to the use of different emission coefficients in the first, upper partial region and the second, lower partial region for the inner-housing outer surface involves a material in the first partial region having a lower emission coefficient than another material which, in this case, however, is applied to the inner-housing outer surface in the second partial region. This makes it possible to continue to employ the materials which have previously been used for the inner and outer housings. The material to be applied has a higher emission coefficient than the inner housing. In this way, a desired, positive radiation effect can be intensified. Preferably, the material to be applied is an oxide ceramic, e.g. zirconium oxide. Furthermore, other coating materials with a suitable radiation property and ability to bond to the material of the housing may also be used. A coating material is preferably also resistant to corrosion in steam. The layer thickness in which the coating material is applied may, for example, be in the range between 50 mm and 100 mm. On the one hand, this offers a particularly high emission coefficient, for example of e=0.8 or higher. On the other hand, the oxide ceramic can be reliably applied to conventional housing material, for example GGG-40, in such a manner that it is able to provide a long service life. One example of a suitable technique for applying a thin film of the oxide ceramic is plasmaspraying. Furthermore, the application method, as well as the oxide ceramic itself, ensure that high chemical resistance to the media which are found in the turbine housing is also provided. In this case, the coating material preferably has a coefficient of thermal expansion which is also suitable, with regard to transient temperature states, for keeping the risk of the housing material flaking at a low level.
A further design aimed at achieving a lower heat transfer through radiation in a part of the first partial region of the inner-housing outer surface to the opposite outer-housing surface compared to a part of the second partial region of the inner-housing outer surface is achieved by the fact that at least a part of the second partial region of the outer-housing inner surface has a greater absorption coefficient than a part of the first partial region of the outer-housing inner surface. This likewise makes it possible to introduce an increased amount of heat into the second, lower partial region of the outer housing. This in turn, once again, evens out the temperatures of the outer housing. Since, as a result, the driving temperature gradient for natural convection between the inner housing and the outer housing is reduced, this design also counteracts natural convection.
A refinement of the configuration of the second partial region with a higher absorption coefficient has a third material in the second partial region on the outer-housing inner surface. This third material has a higher absorption coefficient than a fourth material of the outer-housing inner surface in the first partial region. This third material is either the material of the outer-housing inner surface in the second partial region itself, in which case its surface has been suitably machined, or alternatively the third material may be an additional material which has been applied to the outer-housing inner surface in the second partial region. A further possibility for changing the absorption coefficients between the first, upper partial region and the second, lower partial region of the outer-housing inner surface consists in altering the outer-housing inner surface in the first, upper partial region in such a way that it has a lower absorption coefficient than the outer-housing inner surface of the second, lower partial region.